Synthesis of Donor-Acceptor Substituted Molecular Caltrops: Strategic Use of an AB3 Synthon Based on a Tetraphenylmethane Core

Article abstract of DOI:10.1055/s-2003-42119

Using a tetraphenylmethane based AB₃ tecton, a facile synthetic route to donor-acceptor substituted unsymmetrical molecular caltrops is described. The former compound was readily prepared in two steps from the cheap commercial dye New Fuchsin.

Full text of DOI:10.1055/s-2003-42119

LETTER
2329
Synthesis of Donor–Acceptor Substituted Molecular Caltrops:
Strategic Use of an AB3 Synthon Based on a Tetraphenylmethane Core
Synthesis of
C
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D
yads Ba
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etraphen
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ane Co
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Department of Chemistry, Jadavpur University, Kolkata 700 032, India
Fax +91(33)24146266; E-mail: jusaumitra@yahoo.co.uk
Received 29 July 2003
Symmetrical molecular caltrops such as 2 (Figure 1) have
been the focus of many recent studies.4 They are potential-
ly useful in atomic force microscopy, photovoltaic cells,
as the emitting layer in organic light emitting diodes
(OLEDs) and in the preparation of dendritic light harvest-
ing antennae and star-shaped multiredox systems. Rigid
tetrahedral scaffolds such as tetraphenylmethane and to a
lesser extent, 1,3,5,7-tetraphenyl adamantane (extended
tetrahedron) are used for their synthesis.5 On the other
hand, synthesis of unsymmetrical molecular caltrops viz.
1 has received little attention.6 The reported synthetic pro-
cedures are either target specific or involve cumbersome
synthetic steps and therefore, lack a general synthetic ap-
peal. The absence of a readily available centrally tetrahe-
dral AB3 synthon appears to be the major hurdle towards
this end. In continuation of our interest in tetraphenyl-
methane based novel molecular architectures,7 we now
describe a general synthetic route to unsymmetrical mo-
lecular caltrops via the key use of a tetraphenylmethane
based AB3 tecton 5.
Abstract: Using a tetraphenylmethane based AB3 tecton, a facile
synthetic route to donor-acceptor substituted unsymmetrical mole-
cular caltrops is described. The former compound was readily pre-
pared in two steps from the cheap commercial dye New Fuchsin.
Key words: molecular caltrops, dyad, tetraphenylmethane, Sono-
gashira coupling, Heck reaction
Photoinduced electron transfer (PET) plays a key role in
natural photosynthesis.1 In order to understand this pro-
cess at a molecular level, many covalently bound donor-
acceptor dyads have been investigated in recent years.2,3
The efficiency of PET reaction in these dyads was found
to be highly dependent on the distance and angles between
the donor and the acceptor chromophores. In order to con-
trol these elements, a variety of spacer units, differing in
length and/or topology, have been used between the donor
and the acceptor units. It has been shown that dyads hav-
ing flexible matrices can create problems in PET reac-
tions. Due to their conformational freedom, such dyads
can give rise to undesired associative interactions such as
excimer formation, self-quenching, etc., which may se-
verely interfere with the PET reaction. These problems
can be minimized if a conformationally rigid spacer is
used between the donor and the acceptor chromophores.
We therefore undertook a program on the synthesis and
PET reaction studies of a series of topologically rigid
donor-acceptor dyads and in this paper, describe a facile
synthetic route to unsymmetrical caltrop-shaped dyads 1
(Figure 1) in which the donor and the acceptor units are
held in a rigid tetrahedral arrangement.
Our synthesis started with the cheap commercial dye New
Fuchsin 3 which via threefold diazotization (NaNO2, di-
lute H2SO4, 0 °C) and iododediazoniation reaction (KI,
H2O, r.t.) produced the triiodoaryl carbinol 4 (40%) as de-
scribed before (Scheme 1).8 Acid catalyzed reaction of 4
Figure 1
SYNLETT 2003, No. 15, pp 2329–2332
Advanced online publication: 21.11.2003
DOI: 10.1055/s-2003-42119; Art ID: D19303ST.pdf
© Georg Thieme Verlag Stuttgart · New York
0
1
.1
2
.2
0
0
3
Scheme 1 Reagents and conditions: (i) dilute H2SO4, NaNO2, 0 °C
then KI, H2O, r.t.; (ii) phenol (4 equiv), concd H2SO4, 80 °C.

2330
S. Sengupta et al.
LETTER
with excess phenol at 80 °C then led to 5, the key AB3 tec- then removed (NaOH, THF, r.t.) to give the terminally
ton, in 85% yield.9 The para-isomer was obtained exclu- free tris-acetylene product 8 (80%). The latter may be fur-
sively in this Friedel–Crafts type reaction. Towards ther engaged in Pd-catalyzed couplings with aryl halides
synthesis of unsymmetrical molecular caltrops, we envis- and triflates. However, attempted Sonogashira reaction of
aged that three donor chromophores could be introduced 5 with ethyl propiolate failed to give the desired coupling
into 5 via threefold Sonogashira or Heck reaction on the product. Instead, we obtained the product of the Michael
iodoaryl tripod. The acceptor chromophore could be sub- addition of the phenol to the activated triple bond. The
sequently introduced via the phenolic end. Since the Pd- phenolic group in 6 and 8 were then reacted with para-ni-
catalyzed reactions would lead to either sp- or sp2 hybrid- trobenzyl bromide (K2CO3, CH3CN, reflux) to produce
ized linkers, it was expected that the topological rigidity the donor-acceptor substituted unsymmetrical caltrops 9
of the tripod would remain unaffected.
and 10 in good yields.12 Here, the choice of nitrobenzene
as the acceptor stemmed from two major considerations:
firstly, nitrobenzene is an excellent electron acceptor
(EA/A– ™–1.2 eV) and secondly, its absorption maxima
(a strong K-Band at 250–270 nm and a weak R-band at
300–320 nm) lie in the shorter wavelength side which
We first investigated the threefold Sonogashira reactions
on 5 using phenyl acetylene and TMS-acetylene [10%
PdCl2(PPh3)2, 10% CuI, Et3N, DMF, r.t.]10 which gave
rise to the tris-alkynylated products 6 (66%) and 7 (75%),
respectively (Scheme 2).11 The TMS-groups from 7 were
Scheme 2 Reagents and conditions: (i) 10% PdCl2(PPh3)2, 5% CuI, Et3N, DMF, r.t.; (ii) NaOH, THF, r.t.; (iii) para-nitrobenzyl bromide,
K2CO3, CH3CN, reflux; (iv) 10% Pd(OAc)2, Bu4NBr, KOAc, DMF, 100 °C.
Synlett 2003, No. 15, 2329–2332 © Thieme Stuttgart · New York

LETTER
Synthesis of Caltrop Shaped Dyads Based on a Tetraphenylmethane Core
2331
Photochem. 1991, 16, 1..(d) Kavarnos, G. J. Fundamentals
of Photoinduced Electron Transfer; VCH: Weinheim, 1993..
(e) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res.
1993, 26, 198..(f) Häder, D.-P. Photosynthese; Thieme:
Stuttgart, 1999.
would interfere least with the absorption bands from the
tolan or stilbenoid donors. For example, the absorption
spectrum of 9 in CHCl3 showed strong absorptions at 295
nm (lmax) and 315 nm with shoulders at 280 and 305 nm.
The strong bands are due to the tolan donors (absorption
maxima of 6: lmax 294 nm and 314 nm) and are distinctly
different from the shoulders due to the nitrobenzene ac-
ceptor. Moreover, the band positions of the individual do-
nor and acceptor units remained unaltered in 9 indicating
that there are no ground state interactions between the two
types of chromophores in the dyad. This was much ex-
pected since in a molecular caltrop (1 or 2), the central
sp3-hybridized carbon would disrupt any conjugation
whatsoever between the attached chromophores.
(2) (a) Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J.
Am. Chem. Soc. 1997, 119, 974. (b) Jolliffe, K. A.;
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Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123,
2607. (d) Sakomura, M.; Lin, S.; Moore, T. A.; Moore, A.
L.; Gust, D.; Fujihira, M. J. Phys. Chem. A 2002, 106, 2218;
and references cited therein.
(3) (a) Vögtle, F.; Plevoets, M.; Nieger, M.; Azzellini, G. C.;
Credi, A.; De Cola, L.; De Marchis, V.; Venturi, M.;
Balzani, V. J. Am. Chem. Soc. 1999, 121, 6290. (b) Ceroni,
P.; Vicinelli, V.; Maestri, M.; Balzani, V.; Müller, W. M.;
Müller, U.; Hahn, U.; Osswald, F.; Vögtle, F. New J. Chem.
2001, 25, 989.
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W. J. Jr.; Lahicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc.
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R. A. Jr.; Bazan, G. C. J. Am. Chem. Soc. 2000, 122, 5695.
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Mater. 2000, 12, 1701. (g) Constable, E. C.; Eich, O.;
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Müller, T. J. J. Eur. J. Org. Chem. 2000, 3305. (i) Aujard,
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Meerholz, K. J. Chem. Soc., Perkin Trans. 2 2001, 964.
(k) Yeh, H.-C.; Lee, R.-H.; Chan, L.-H.; Lin, T.-Y. J.; Chen,
C.-T.; Balasubramaniam, E.; Tao, Y.-T. Chem. Mater. 2001,
13, 2788. (l) Maus, M.; De, R.; Lor, M.; Weil, T.; Mitra, S.;
Wiesler, U. M.; Herrman, A.; Hofkens, J.; Vosch, T.;
Müllen, K.; De Schryver, F. C. J. Am. Chem. Soc. 2001, 123,
7668. (m) Weil, T.; Wiesler, U. M.; Herrman, A.; Bauer, R.;
Hofkens, J.; De Schryver, F. C.; Müllen, K. J. Am. Chem.
Soc. 2001, 123, 8101. (n) Zimmerman, T. J.; Müller, T. J. J.
Synthesis 2002, 1157. (o) Zimmerman, T. J.; Müller, T. J. J.
Eur. J. Org. Chem. 2002, 2269. (p) Summers, M. A.;
Robinson, M. R.; Bazan, G. C.; Buratto, S. K. Chem. Phys.
Lett. 2002, 364, 542. (q) Li, Q.; Rukavishnikov, A. V.;
Petukhov, P. A.; Zaikova, T. O.; Keana, J. F. W. Org. Lett.
2002, 4, 3631. (r) Grimsdale, A. C.; Bauer, R.; Weil, T.;
Tchebotareva, N.; Wu, J.; Watson, M.; Müllen, K. Synthesis
2002, 1229. (s) Sengupta, S.; Purkayastha, P. Org. Biomol.
Chem. 2003, 1, 436; and references cited therein.
In order to introduce stilbenoid donors, a threefold Heck
reaction of 5 was carried out with styrene under Jeffery’s
PTC- conditions [cat. Pd(OAc)2, Bu4NBr, KOAc, DMF,
100 °C)]13 which led to the tristilbenoid phenol 11 in 83%
yield (Scheme 2).14 However, 11 was found to be sparing-
ly soluble in common organic solvents. This solubility
problem was solved by using para-tert-butyl styrene for
the Heck reaction which produced the soluble tristilbenyl
donor 12 in 75% yield.14 The latter was then coupled with
para-nitrobenzyl bromide, as before (K2CO3, CH3CN, re-
flux), to produce the stilbene-nitrobenzene molecular cal-
trop 13 (32%) as a pale yellow solid.12 The low yield of 13
was due to its poor solubility in the reaction medium
which led to incomplete conversion. The absorption spec-
trum of 13 in CHCl3 showed a broad maximum at 320 nm
with a shoulder at 328 nm. The former was due to the stil-
benoid donors (absorption maxima of 12: 318 nm) and
was slightly red shifted from that of trans-stilbene itself.15
Presumably, a small degree of homoconjugation is present
in 12 and 13 which operates through their central sp3-hy-
bridized carbons.4d,e,7d The shoulder at 328 nm was due to
the R-band of the nitrobenzene acceptor. Again, no
ground state interaction between the stilbene and the ni-
trobenzene chromophores was evident in 13.
In conclusion, a facile synthetic route to (donor-acceptor
substituted) unsymmetrical molecular caltrops is de-
scribed via the key intermediacy of a centrally tetrahedral
AB3 tecton 5. Further use of 5 in the synthesis of un-
symmetrical caltrops can be envisaged via threefold Pd-
catalyzed couplings on the iodoaryl tripod, conversion of
the phenol to a triflate and further Pd-catalyzed reactions
at the latter end.
(5) For less common centrally tetrahedral scaffolds viz. 9,9¢-
spirobifluorene and tetraphenylsilane, see: (a) Tour, J. M.
Chem. Rev. 1996, 96, 537. (b) Salbeck, J.; Yu, N.; Bauer, J.;
Weissortel, F.; Bestgen, H. Synth. Met. 1997, 91, 209.
(c) Baurele, P.; Mitschke, U.; Mena-Osteritz, E.;
Acknowledgment
Professor Nitin Chattopadhyaya is warmly thanked for some
absorption measurements.
Sokolowski, M.; Muller, D.; Grop, M.; Meerholz, K. Proc.
SPIE-Int. Soc. Opt. Engl. 1998, 3476, 32. (d) Pei, J.; Ni, J.;
Zhou, X.-H.; Cao, X.-Y.; Lai, Y.-H. J. Org. Chem. 2002, 67,
4924. (e) Chan, L.-H.; Lee, R.-H.; Hsieh, C.-F.; Yeh, H.-C.;
Chen, C.-T. J. Am. Chem. Soc. 2002, 124, 6469. (f) Also
see ref.4e
References
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Benjamin-Cumming: Menlo Park CA, 1978..(b) Fox, M.
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Amsterdam, 1988..(c) Gust, D.; Moore, T. A. Adv.
Synlett 2003, No. 15, 2329–2332 © Thieme Stuttgart · New York

2332
S. Sengupta et al.
LETTER
(6) (a) Yao, Y.; Tour, J. M. J. Org. Chem. 1999, 68, 1968.
(b) Hirayama, D.; Takiyama, K.; Aso, Y.; Otsubo, T.;
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Am. Chem. Soc. 2002, 124, 7801. (d) Piotrowiak, P.;
Galoppini, E.; Wei, Q.; Meyer, G. J.; Weiwiór, P. J. Am.
Chem. Soc. 2003, 125, 5278. (e) Li, Q.; Rukavishnikov, A.
V.; Petukhov, P. A.; Zaikova, T. O.; Jin, C.; Keana, J. F. W.
J. Org. Chem. 2003, 68, 4862. (f) Jian, H.; Tour, J. M. J.
Org. Chem. 2003, 68, 5091.
(7) (a) Sengupta, S.; Sadhukhan, S. K. Tetrahedron Lett. 1998,
39, 1237. (b) Sengupta, S.; Sadhukhan, S. K. Tetrahedron
Lett. 1999, 40, 9157. (c) Sengupta, S.; Sadhukhan, S. K.
Organometallics 2001, 20, 1889. (d) Sengupta, S.;
Sadhukhan, S. K.; Muhuri, S. Tetrahedron Lett. 2002, 43,
3521. (e) Sengupta, S.; Sadhukhan, S. K.; Singh, R. S.
Indian J. Chem., Sect. B 2002, 41, 642. (f) Sengupta, S.;
Sadhukhan, S. K. Indian J. Chem., Sect. B 2003, 42, 858.
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1997. (b) Sengupta, S.; Sadhukhan, S. K. J. Chem. Soc.,
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(9) Compound 5: mp 206–207 °C (MeOH). IR (KBr): 3400,
1605, 1500, 1460, 1370 cm–1. 1H NMR (300 MHz, CDCl3/
TMS): d = 2.33 (s, 9 H), 6.65 (dd, J = 8.4, 2.1 Hz, 3 H), 6.70
(d, J = 6.6 Hz, 2 H), 6.99 (d, J = 6.6 Hz, 2 H), 7.02 (d, J = 2.1
Hz, 3 H), 7.65 (d, J = 8.4 Hz, 3 H). 13C NMR (75 MHz,
CDCl3): d = 28.2, 63.3, 98.7, 114.5, 130.1, 131.9, 138.0,
140.5, 146.5, 153.6.
(10) (a) Sonogashira, K. In Comprehensive Organic Synthesis,
Vol. 3; Trost, B. M.; Fleming, I., Eds.; Pergamon Press:
Oxford, 1991, 551. (b) Sonogashira, K. In Metal-catalyzed
Cross-coupling Reactions; Diederich, F.; Stang, P., Eds.;
Wiley-VCH: Weinheim, 1998, 203.
2.35 (s, 9 H), 3.26 (s, 3 H), 6.71 (d, J = 8.7 Hz, 2 H), 6.93 (d,
J = 8 Hz, 3 H), 7.00 (d, J = 8.7 Hz, 2 H), 7.01 (s, 3 H), 7.33
(d, J = 8 Hz, 3 H). 13C NMR (75 MHz, CDCl3): d = 20.9,
64.0, 80.9, 82.3, 114.5, 119.6, 128.3, 131.7, 132.1, 138.0,
139.9, 147.0, 153.7.
(12) Compound 9: mp 130–132 °C (MeOH). IR (KBr): 2916,
1596, 1521, 1500, 1442, 1344 cm–1. 1H NMR (300 MHz,
CDCl3/TMS): d = 2.36 (s, 9 H), 5.08 (s, 2 H), 6.78 (d, J = 9
Hz, 2 H), 6.92 (dd, J = 8.1, 2 Hz, 3 H), 7.01 (d, J = 2 Hz, 3
H), 7.06 (d, J = 9 Hz, 2 H), 7.25–7.33 (m, 11 H), 7.42–7.46
(m, 6 H), 7.54 (d, J = 8.1 Hz, 3 H), 8.18 (d, J = 9 Hz, 2 H).
13C NMR (75 MHz, CDCl3): d = 21.4, 64.5, 69.1, 88.5, 93.8,
114.2, 121.2, 123.9, 124.2, 128.0, 128.5, 128.7, 128.9,
131.3, 131.4, 131.9, 132.2, 132.6, 139.5, 139.8, 144.8,
147.0, 156.7.
Compound 10: mp 132–135 °C (MeOH). IR (KBr): 2900,
2100, 1590, 1518, 1487, 1350 cm–1. 1H NMR (300 MHz,
CDCl3/TMS): d = 2.36 (s, 9 H), 3.24 (s, 3 H), 5.09 (s, 2 H),
6.73 (d, J = 9 Hz, 2 H), 6.91 (dd, J = 8, 1.8 Hz, 3 H), 7.00 (d,
J = 1.8 Hz, 3 H), 7.06 (d, J = 9 Hz, 2 H), 7.30–7.36 (m, 5 H),
8.08 (d, J = 8.7 Hz, 2 H).
Compound 13: mp 203–205 °C. IR (KBr): 2902, 1604,
1521, 1508, 1456, 1344, 1242 cm–1. 1H NMR (300 MHz,
CDCl3/TMS): d = 1.33 (s, 27 H), 2.36 (s, 9 H), 5.16 (s, 2 H),
6.87 (d, J = 9 Hz, 2 H), 7.00 (d, J = 16.2 Hz, 3 H), 7.05–7.12
(m, 6 H), 7.24 (d, J = 9 Hz, 2 H), 7.28 (d, J = 16.2 Hz, 3 H),
7.35–7.53 (m, 14 H), 7.62 (d, J = 9 Hz, 3 H), 8.25 (d, J = 9
Hz, 2 H). 13C NMR (75 MHz, CDCl3): d = 20.7, 31.7, 35.0,
64.1, 69.0, 114.0, 124.2, 124.4, 124.8, 125.8, 126.0, 126.6,
128.0, 129.5, 129.9, 130.3, 132.6, 133.1, 134.4, 135.1,
135.4, 140.5, 145.0, 146.4, 156.5.
(13) (a) Jeffery, T. Tetrahedron 1996, 52, 10113. (b) de Meijere,
A.; Bräse, S. In Metal-catalyzed Cross-coupling Reactions;
Diederich, F.; Stang, P., Eds.; Wiley-VCH: Weinheim,
1998, 99. (c) Beletskaya, I. P.; Cheprakov, A. V. Chem. Rev.
2000, 100, 3009.
(14) Compound 11: mp 201–202 °C. IR (KBr): 3290, 1595,
1500, 1445, 1260 cm–1. 1H NMR (300 MHz, CDCl3/TMS):
d = 2.35 (s, 9 H), 6.73 (d, J = 8.7 Hz, 2 H), 7.00 (d, J = 16.2
Hz, 3 H), 7.05–7.11 (m, 6 H), 7.14 (d, J = 8.7 Hz, 2 H), 7.20–
7.31 (m, 6 H), 7.34 (d, J = 5.4 Hz, 3 H), 7.35 (d, J = 16.2 Hz,
3 H), 7.45–7.54 (m, 9 H). 13C NMR (75 MHz, CDCl3): d =
20.2, 63.7, 114.3, 124.4, 126.1, 126.5, 127.4, 128.6, 129.1,
129.7, 132.2, 132.7, 133.7, 134.7, 137.8, 139.2, 146.3,
153.4.
(11) Representative Procedure for Three-fold Sonogashira
Couplings on 5: PdCl2(PPh3)2 (5 mg) was added to a
degassed solution of 5 (0.10 g, 0.13 mmol), phenyl acetylene
(0.08 g, 0.8 mmol) and CuI (4 mg) in a mixture of DMF (3
mL) and Et3N (2 mL). The reaction mixture was stirred at r.t.
for 16 h. It was then concentratedd under reduced pressure,
diluted with water and extracted with CH2Cl2. The organic
layer was dried and the solvent removed under reduced
pressure. The residue was purified by silica gel
chromatography (10% EtOAc in light petroleum) to give 6
(0.057 g, 66%) as a white solid; mp 126–127 °C (CHCl3–
MeOH). IR (KBr): 3410, 1600, 1520, 1465, 1360 cm–1. 1H
NMR (300 MHz, CDCl3/TMS): d = 2.43 (s, 9 H), 6.73 (d, J
= 8.6 Hz, 2 H), 6.90–7.16 (m, 8 H), 7.29–7.48 (m, 12 H),
7.49–7.62 (m, 6 H). 13C NMR (75 MHz, CDCl3): d = 21.0,
64.5, 88.5, 93.8, 114.5, 121.2, 123.9, 125.6, 128.3, 128.7,
128.9, 130.1, 131.9, 132.2, 138.0, 139.5, 147.0, 156.5. 8: mp
105–106 °C (MeOH). IR (KBr): 3290, 3000, 2910, 2090,
1595, 1480 cm–1. 1H NMR (300 MHz, CDCl3/TMS): d =
Compound 12: mp 220–224 °C (CHCl3–MeOH). IR (KBr):
3300, 1600, 1510, 1440, 1280 cm–1. 1H NMR (300 MHz,
CDCl3): d = 1.33 (s, 27 H), 2.34 (s, 9 H), 4.67 (br s, 1 H),
6.72 (d, J = 8.5 Hz, 2 H), 6.98 (d, J = 16 Hz, 3 H), 7.04–7.11
(m, 6 H), 7.14 (d, J = 8.3 Hz, 2 H), 7.26 (d, J = 16 Hz, 3 H),
7.37 (d, J = 8.2 Hz, 6 H), 7.45 (d, J = 8.8 Hz, 6 H), 7.48 (d,
J = 9.2 Hz, 3 H).
(15) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399.
Synlett 2003, No. 15, 2329–2332 © Thieme Stuttgart · New York